This invention relates to the covert identification of genuine and counterfeit articles and has particular (but not exclusive) relevance to the retail trades.
UK patent no. 2302308 discloses a technique for protecting articles using a covert labelling system, which can be verified by ostensibly covert means. The technique relies on the incorporation of an optical filter on the article (or its packaging), which is then verified or authenticated by observation using a second filter. Typically, the transmittance versus wavelength characteristic of each of the optical filters exhibits at least one peak or trough. The design of the second filter is such that it is complementary to the first and the combination produces an optical effect that is absent in either of the two individual filters.
UK patent no. 2302308 describes a variety of optical filters suitable for use with the above technique including conventional thin film filters based on Fabry-Perot etalons or distributed Bragg reflectors. The structures of these devices are well known to those skilled in the art; see for example, U.S. Pat. No. 4,756,602, which discusses a conventional all-dielectric Fabry-Perot etalon design.
The optical characteristics of the filters may be enhanced by employing more complex filter structures, for example using filters based on coupled Fabry-Perot etalons or multiple cavity Fabry Perot etalons. Again, the structures of such devices are well known to those skilled in the art.
For example, U.S. Pat. No. 4,756,602 extends the idea of a single all dielectric Fabry-Perot etalon to disclose a narrow band-pass filter, exhibiting a single transmission peak, based on a conventional coupled cavity Fabry-Perot etalon. Similarly, U.S. Pat. No. 5,410,431 discusses a staggered coupled cavity design suitable for use in the above technique, whilst UK patent GB 1306335 describes another potential filter configuration using a multiple Fabry-Perot etalon design.
UK patent GB2302308 suggests other interesting optical effects are possible using the aforementioned technique, for example modulating the reflection characteristics of the label as a function of angular viewing position. GB 1270042 discloses a multilayer bandpass filter which ensures that the pass-band exists only for a single angle of incidence.
Other examples described in UK 2302308 are based on high transmission comb filters which can provide an essentially neutral spectral characteristic. However, the total thickness of material required for the above filters can be as much as 10xc3x9710xe2x88x926 m, which would place a significant obstacle to their realisation using standard industrial production techniques. Also, the effective range of wavelengths over which simple all-dielectric Fabry-Perot filters operate is somewhat limited by unwanted sidebands of transmission which appear on either side of the peak in the passband. Referring to the transmission versus wavelength characteristic for a particular filter, the range of wavelengths bounded by the unwanted sidebands governs the practical operating range of the filter. This range is defined as the locus of minimum transmission and varies from filter to filter according to the filter configuration. The narrow loci of minimum transmission exhibited by the above filters has the effect of reducing the range of wavelengths over which the filters can be used in practice.
In addition, the net integrated photopic transmission of the aforementioned filters is often low, reducing the brightness of the optical effect produced by the technique and restricting the range of colour effects achievable.
The present invention relates to an improved optical filter design for use in the covert identification of genuine and counterfeit articles. It has particular relevance to the retail trades where it offers an alternative to the known optical filters used in conventional anticounterfeiting techniques. A significant advantage of the design is that the optical filter exhibits a total film thickness of less than 10xe2x88x926 m, thereby improving industrial production techniques compared to conventional filters. In addition, the optical filter exhibits a broad locus of minimum transmission, enabling operation over a wide range of wavelengths.
The net integrated photopic transmission of the filter is high, increasing the range of colour effects available and producing a bright optical effect which is easily perceived by the unaided eye.
According to the present invention, an optical filter comprises a resonantly coupled Fabry-Perot etalon, having at least two component cavities, wherein each component cavity comprises one set of multiple half wave spacer layers between two partially reflecting mirrors,
characterised by the addition of a single layer of multiple quarter wave thickness to one of the cavities, said additional layer having a contrasting refractive index to that of the spacer.
Preferably the partially reflecting mirrors comprise thin metallic layers or quarter wave stacks of dielectric materials.
In a preferred embodiment the locus of minimum transmission extends over substantially the whole of the visible part of the electromagnetic spectrum.
In particular, the locus of minimum transmission extends over the range of wavelengths 400 nm-700 nm.
In a further preferred embodiment the transmittance versus wavelength characteristic contains at least one pass band in the near infrared region of the electromagnetic spectrum.
Preferably the net integrated photopic transmission of the filter exceeds 25%.
In a second aspect of the present invention, a method of marking an article so that its authenticity can be determined when viewed through a first optical filter means comprising an optical filter according to the above description, comprises the step of incorporating into the article or its packaging, a second optical filter means comprising an optical filter according to the above description, wherein the second optical filter means is adapted such that when viewed through the first optical filter means a strong change in colouration is observed.
Preferably the optical filter means are arranged to be essentially colourless in their individual general appearance but to take on a strong colouration when viewed together in transmission.
In a preferred embodiment the transmittance versus wavelength characteristics of the first and second optical filter means are arranged to be substantially complementary, such that the second optical filter means appears essentially black when viewed through the first optical filter means.
In a further preferred embodiment the transmittance versus wavelength characteristics of the first and second optical filter means are arranged to be substantially similar at selected wavelengths such that the second optical filter means exhibits a strong change in colour hue when viewed through the first optical filter means.
The second optical filter means may be incorporated into a paint or a label which is applied to the article or its packaging.
In another embodiment the second optical filter means is deposited on a polymer film which is subsequently applied to the article, to its packaging or to a label attached to the article or its packaging.
At least one of the optical filter means may be derived from a plurality of optical filters, the filters being deposited on layers of polymer film which are subsequently laminated together to produce the optical filter means.
In one preferred arrangement both the first and the second optical filter means are applied to the article, its packaging or a label attached to the article or its packaging.
The first optical filter means may be incorporated into a pair of spectacles.